A famous person once observed that the signature of a civilized mind is the ability to hold two seemingly contradictory ideas in one’s head at the same time. This is exactly what conservation must learn to do when it comes to introduced (or what we often call “non-native” or “invasive”) species.
- Conservation orthodoxy has it that non-native species are evil or at least highly undesirable, and that we conservationists need to do everything we can to eliminate, prevent and control any and all non-native plants and animals. Up until a few years ago, The Nature Conservancy had an entire global team specifically addressing the problem of invasive species. Today, the Conservancy’s policy team seeks regulations and trade agreements that will reduce the flow of exotic species to new lands and waters.
- Pointing in the opposite direction, an essay appearing this week in the journal Nature — and co-authored by several of the world’s premier ecologists — argues that we should assess organisms on their environmental impact rather than on whether or not they are native. (Did you know that the common pheasant — image above — was an introduced species to North America from Asia in 1857?)
These two views are not as contradictory as the media will portray them — but no one has recently accused the media of having a civilized mind.
Start holding these ideas together in your head:
1. Non-native species can have devastating impacts, especially on islands. Exotic pests (such as the emerald ash borer) and plant pathogens cause enormous economic damage. Some plant invaders totally remake ecosystems — altering fire regime, nutrient cycling, productivity and resident animals. Some introduced animals have devastated fish, bird and lizard diversity.
2. But non-native plants can also provide habitat for endangered species, and when landscapes have been trashed, exotic trees with rapid growth can be the best bet for erosion and mudslide control.
3. Trade restrictions and border inspections substantially (and relatively cost-effectively) reduce the flood of non-native pests, and some eradication programs have succeeded. For example, on Santa Cruz Island (off the coast of southern California), the Conservancy has eliminated pigs and in the process greatly enhanced the prospects for the highly endangered island fox.
4. But millions of dollars have been spent by federal agencies to control certain invasive species that have no measurable impacts on biodiversity or ecosystem function, and that in the end are not even reduced by the investment — in other words, lots of money has been wasted.
5. It is important to protect ecosystems from certain invaders that could radically change the communities and overwhelm native species.
6. But we must admit that novel ecosystems are increasingly common and these may contain non-native species that will have to be treated as “part of the ecosystem” going forward.
Some of my colleagues in conservation will label the Nature article by Mark Davis and eighteen other ecologists “dangerous” or “destructive” because it puts forth the even-numbered ideas above. It will be interpreted as undermining existing programs to prevent or control invasive species. I do not view it that way. I see it as a provocative essay intended to make us think about how we invest our limited conservation energy and funds.
Science-based conservation cannot be about knee-jerk platitudes and simple views of good and evil. Policy experts and conservationists who have been working hard to control invasive species should not discourage arguments about invasive species — the fact is we cannot control all invasive species, and in many cases, yesterday’s invaders have become plants and animals that are beloved by local people. The concept of “nativeness” did not even really appear in the literature until the mid-19th century, the construct of the British botanists John Henslow and (later) Hewitt (H.C.) Watson.
We — both in the Conservancy and in the broader conservation world — need to take seriously the challenges issued by Mark Davis and his colleagues, and think about what they mean for where we invest our money. Triage is an act of responsibility, not surrender. I am confident that The Nature Conservancy’s investment in eradicating pigs from Santa Cruz Island was smart. I am equally confident that polices restricting the flow of non-native plants into the country could reduce future economic damage from non-native insects and pathogens. But I have also seen money spent on futile invasive control programs or on targeting non-natives that are relatively inconsequential.
I once co-authored a paper about non-native species entitled “Reducing the risks of nonindigenous species introductions: Guilty until proven innocent.” Upon reading the article by Mark Davis, I would amend that title with: “Guilty, but take into account special circumstances when sentencing.”
(Image: Common or Ring-necked pheasant. Image credit: matt knoth/Flicker through a Creative Commons license.)
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